1. Field of the Invention
This invention generally relates to fluid flow measurements and more specifically to an electromagnetic flowmeter and a method for measuring unsteady fluid flow.
2. Statement of Prior Art
Flowmeters of different types have been used for measurements of flow of fluids under steady state conditions. However, they are inherently not applicable to transient flow measurements. As an example, turbine flowmeters are momentum devices. Even though they have a fast signal response time and consequently provide output at an apparently high data rate, the meter calibration is inherently a function of not only velocity profile but also of change of flow rate. Besides, steady-state flowmeters of head type such as orifice plates, are dependent on the response of the pressure transducers used in the differential pressure measurement and the flow field in the vicinity of the orifice can change considerably under high flow transients leading to changes in the orifice's calibration and inaccurate measurements.
Another type of fluid flowmeter commonly used for steady-state flow rate is an electromagnetic flowmeter. In a typical electromagnetic fluid flowmeter, a magnetic field is introduced into the fluid flow. As an electrically conducting fluid flows through the magnetic field, an electromotive force (e.m.f) is generated according to Faraday's Law of Electromagnetism. The e.m.f. generated is proportional to the fluid velocity and is perpendicular to both the electric current vector or the velocity vector of the fluid and the magnetic flux density vector. A measure of the e.m.f. and any variations thereof are proportional to the corresponding variations in the velocity of the fluid provided the magnetic field in the region of interest is kept constant.
Several transient flowmeters have been developed for specific applications based upon commercially available electromagnetic flowmeters for steady-state fluid flow rate conditions. A few special-purpose meters have been developed over the past 25 years that have had some transient flow measurement capabilities. In 1950, Arnold described an electromagnetic flowmeter for small flow rates that responded to very short transients of 0.001 second in duration. Since a circular cross section proved unsatisfactory for the design, a small rectangular cross section of 0.48.times.1.58 cm was chosen.
In 1960, Iwanicki and Fontaine described an electromagnetic flowmeter, which was also designed to respond to very short transients of low flow rates. The meter, with a 1.2-cm diameter, could not be operated for more than a few seconds due to polarization of the flowing liquid at the signal electrodes. In each case, it was shown that the meters responded to large flow transients. Unfortunately, the facilities used to validate meter performance could not provide information on the accuracy of the flowmeter output. As described partially in my article with W. Durgin: A Transient Electromagnetic Flowmeter and Calibration Facility, Journal of Fluids Engineering, March 1990, p. 12-15, which is incorporated herein in its entirety
When steady-state electromagnetic fluid flowmeters are adapted to operate as transient flowmeters, the process, it involves re-design of the electronic required to analyze the transient signal from the transient fluid flowmeter. Many attempts have been made with some success. However, there is still a need for an accurate and a simple mcthod which can be used to measure the transient rate of flow of a fluid.